Ultra-wide bandwidth power amplifiers that can, for instance, produce 100-watt microwave outputs in the past have utilized traveling wave tubes. One application for such high-power microwave traveling wave tube amplifiers is in electronic warfare to countermeasure radar-based seekers by emitting jamming signals. In many of these applications a traveling wave tube amplifier is towed in a vehicle behind an aircraft and is provided with a high-voltage source to run the traveling wave tube. Control and modulating signals are also coupled to the towed vehicle for jamming and decoy purposes.
Typical operational requirements include the ability to produce high-power microwave signals from 2 GHz to 18 GHz to accommodate a variety of threats.
However, there are serious limitations in such decoys or jammers due to the use of the traveling wave tubes themselves. First and foremost, the traveling wave tubes are unreliable, difficult to manufacture, and are scarce at best. Secondly, these tubes require high voltages, which for a towed vehicle, must be supplied inside an optical fiber towline that must have high tensile strength and flexibility; be lightweight; and capable of being spooled without kinking.
Additionally, for jamming or countermeasure purposes, oftentimes it is required to turn the amplifier on and off in nanosecond time frames, which traveling wave tubes cannot accommodate. Moreover, if the traveling wave tube is not left on, it must be powered up, involving considerable warm-up time.
For communications purposes, oftentimes it is required to be able to frequency-hop across multiple octaves of frequency and yet provide sufficient communication power levels. An ultra-wideband high-power amplifier is highly desirable for such applications.
In order to eliminate the problems associated with traveling wave tube microwave power amplifiers, there have been efforts in the past to eliminate the tube and provide solid-state devices. Some have suggested using solid-state amplifiers that employ distributed amplification using a number of small amplifiers in small unit cells that are operated in parallel in an effort to obtain high power. However, for high power applications, the power is limited by the power handling capability of the final active device in the distributed structure. In addition these distributed structures do not provide the total aggregate power available from each of the small unit cells in high power applications (i.e., N unit cells do not provide N×the power per unit cell).
To illustrate, in providing, for instance, 100 watts of power, one would need to have active devices that are relatively large. However, relatively large devices, although available, have ever-increasing parasitics. As a result, when one parallels these additional cells to increase the power level, the parasitics multiply, the impedance Of the device is reduced and the internal cutoff frequency of the distributed amplifier is lowered. All of these factors serve to reduce the power-bandwidth product of the amplifier.
Thus the reasons for the non-use of conventional uniformly distributed amplification in power amplifiers have been the limitations on the size of the transistors to limit parasitic problems, the limited operating frequency range because of the parasitics and the sub-optimal loading of the transistors, and the suboptimal transistor combining for high power applications.
Additionally, one-dimensional uniformly distributed solid-state microwave amplifiers have a gate line termination and a drain line termination. Gate line termination does not affect amplification. However, the drain line termination reduces the power available to the load and thus ultimately limits the output power of the amplifier.
In many applications the use of the drain resistor wastes up to 50% of the power. The reason that one utilizes a termination in the drain line is to eliminate the reverse traveling wave so that the amplifier is well matched over broad frequency bandwidths.
Moreover, there is another problem with prior uniformly distributed amplifiers. In these amplifiers each cell's amplification varies with frequency primarily due to unmatched capacitances. What makes capacitance matching so difficult in conventional distributed amplifiers is that the load line for each cell is different. The problem is that over different parts of the frequency range that one wishes to cover, one device will be optimally loaded and other devices will be sub-optimally loaded. Thus the maximum power output of each cell is not delivered.
Further, each transistor in the uniformly distributed amplifier is typically of a different size with different associated capacitance. This makes internal matching difficult and one has to tailor the matching for each different amplifier. The primary reason for the use of different-size transistors is to handle the output power from a previous cell. In order to handle the high power at the end of the string, these distributed amplifiers use transistors of ever-increasing size. However, this presents different and increasing capacitances.
Because of parasitic capacitance over wide frequency ranges the voltages and currents in each of the cells of the uniformly distributed amplifiers are different, such that each cell would be loaded slightly differently. As mentioned above, the power delivered by each of the transistors would not be maximized. What this means is that, at one part of the frequency band, one transistor would be delivering more power than the other, whereas at a different portion of the frequency band a different transistor would be delivering more power. Thus in a solid-state uniformly distributed microwave amplifier architecture, one could not achieve a flat or fixed output power across multiple octaves in which the power level is equal to the maximum power available from each transistor.
In an effort to solve some of the above problems, U.S. Pat. No. 5,485,118, Non-Uniformly Distributed Power Device, by Richard Chick, issued Jan. 16, 1996, utilizes a two-dimensional array of cells and a very complicated method for distributing the cells. Aside from the complicated requirements for impedance matching, the array of distributed transistors mandates crossover interconnections in both gate and drain circuitry. Because of the crossovers the Chick amplifier is virtually impossible to build utilizing Monolithic Microwave Integrated Circuits (MMICs). This is due to the fact that the interconnection of the cells is always accomplished in one plane. While there are techniques such as an air bridge that permit crossovers, the techniques oftentimes do not have the current-handling capability required for high-power operations. Thus the two-dimensional Chick amplifier was not realizable.
Chick does discuss a one-dimensional, 1-D approach using non-uniform distribution but rejects the 1-D approach in favor of this two-dimensional approach to obtain the broadest theoretical bandwidth. Also, rather than using identical building blocks and coupling them together to achieve the desired amplification, Chick opts for a matrix or grid that requires very complicated reactance compensation between the amplifier tiers or sections. Because of the complicated reactance compensation between amplification stages it was not clear that there could be a simple replication of the distributed amplifier system to achieve high power broadband amplification.
Rather, Chick achieves amplification through arranging a number of devices and adjusting the coupling between the devices in a sensitive tweaking process involving painstaking adjustments to capacitance and inductance associated with each of the cells. Once adjusted, any circuit changes or additions require a further tailored adjustment. Physical layout constraints further complicate this process and limit performance.
Thus Chick does not envision using replicated non-uniform distributed amplifier strings as building blocks coupled together to obtain high power and ultra-wide bandwidth.
Other patents relating to distributed amplifiers are: U.S. Pat. No. 4,733,195, Travelling-Wave Microwave Device, by Hua Q. Tserng et al., issued Mar. 22, 1988; U.S. Pat. No. 4,788,511, Distributed Power Amplifier, by Manfred J. Schindler, issued Nov. 29, 1988; U.S. Pat. No. 4,543,535, Distributed Power Amplifier, by Yalcin Ayasli, issued Sep. 24, 1985; U.S. Pat. No. 4,486,719, Distributed Amplifier, by Yalcin Ayasli, issued Dec. 4, 1984; U.S. Pat. No. 5,046,155, Highly Directive, Broadband, Bidirectional Distributed Amplifier, by James B. Beyer et al., issued Sep. 3, 1991; U.S. Pat. No. 4,754,234, Broadband Distributed Amplifier for Microwave Frequencies, by Patrice Gamand, issued Jun. 28, 1988; and U.S. Pat. No. 5,028,879, Compensation of the Gate Loading Loss for Travelling Wave Power Amplifiers, by Bumman Kim, issued Jul. 2, 1991.
In short, there still remains a need for realizable broad bandwidth power amplifiers in the 2-18 GHz range without utilizing traveling wave tubes.